Remove lib/neutron-legacy leftovers

In [1] we finally got rid of the unfinished lib/neutron module and kept
only lib/neutron-legacy. It's renamed to lib/neutron now and it's the
only neutron related module in Devstack.
So this patch removes leftovers related to the old lib/neutron-legacy.

[1] https://review.opendev.org/c/openstack/devstack/+/865014

Change-Id: I1c61f2184bf3485e985b0165ea472816a7a7c097

Author: slawqo

devstack/upgrade/upgrade.sh

@@ -43,7 +43,6 @@ source $TARGET_DEVSTACK_DIR/lib/nova
 source $TARGET_DEVSTACK_DIR/lib/apache
 source $TARGET_DEVSTACK_DIR/lib/keystone
 source $TARGET_DEVSTACK_DIR/lib/neutron
-source $TARGET_DEVSTACK_DIR/lib/neutron-legacy
 
 
 GENERIC_SWITCH_DEVSTACK_DIR=$(dirname "$0")/..